Sedimentation

Sedminentation can be used to test particulate materials as they settle in a liquid and these principles were defined by Stokes.

George Stokes

Sedimentation is a broad area that can be used to estimate particle sizes in a sample as they settle, in a simple test with a manual Andreasen Pipette or to obtain a distribution with an analytical instrument such as the classic Sedigraph or with a Centrifugal Particle Size Analyser such as the BI-DCP. Gravity systems work well for some materials and these size over the useful 0.3 to 300 micron range. Centrifugation can reduce the range on the fine side, often extending to below 0.05 microns – at a cost in time and sample handling.

Sequoia Scientific are manufacturers of dedicated marine, estuary and aquatic analysers. Their LISST 100ST series are a special class instrument used by marine, lake and river scientists who need to investigate suspended material and its settling in a column of water. The difference is that settling in a trapped column of water is not done to size the sediment but to observe settling and measure the settling velocity distribution of sediments of different sizes. The idea behind this and its use are explained by this article in Coastal Wiki.

Then we have special cases like we are looking to test the stability of suspensions or emulsions against gravity with time. Manufacturers of foods, paints and cosmetics especially work to formulate products for long shelf life’s and predictable performance. Formulaction were founded to address that very market and their Turbiscan range is ideal for the realistic testing of materials.

And of course we use DLS and the Stokes Einstein relationship to test the size of small suspended particles, as with the Zetasizer, and to understand stability and steric and charge stabilisation. Water quality at treatment is a major interest for Zetapotential and the controlled addition of flocculants to aide or accelerate sedimentation. Laser Diffraction is then used in soils and sediment research worldwide to test particle size quickly, automatically and reliably. Sizing by systems like Mastersizer 2000 with an Autosampler has to a large extent replaced Pipette, Sedigraph and Sieving approaches offering dedicated Soils reports, PHI units and broad size ranges from silts to sands.